(1) Field of the Invention
The invention relates to a circuit for protecting a battery, and more particularly, to a circuit for protecting a rechargeable battery wherein a control circuit and a FET switch are integrated onto a single integrated circuit device.
(2) Description of the Prior Art
Rechargeable batteries are used in a variety applications of portable electronic devices. In particular, rechargeable batteries are used for portable phones. Several types of batteries are used in the art, most notably those comprising lithium ion or Li+. For optimum battery life and performance, the battery must be protected from excessive charging voltage during recharging and from over discharging while supplying the appliance.
Referring now to FIG. 1, an exemplary prior art battery protection circuit is shown. A rechargeable battery 10 is configured such that it can source energy to a load 34 or can be recharged by a charger source 38. Two MOSFET switches N118 and N226 are used to control the flow of current into (charging) and out of (discharging) the battery. The control circuit 14 controls the ON and OFF state of the switches 18 and 26. Typically, the switches comprise discrete devices. The control circuit 14 typically comprises a separate circuit from the switches. Each switch 18 and 26 contains a parasitic bulk-to-drain diode D122 and D230. By coupling each switch in an opposite manner, each of the parasitic diodes 22 and 30 conducts current in a different direction.
During normal charging or discharging of the battery 10, both of the switches are ON. However, if the control circuit 14 detects an over charging voltage from VBATT to GND, then the control circuit turns OFF the switch N1. Since the parasitic diode D122 also blocks current flow into the battery, the battery stops charging. In the case of an over discharging condition, the control circuit 14 would detect a too low battery voltage from VBATT to GND. The other switch N226 would be turned OFF. In this case, the parasitic diode D230 blocks current flow out of the battery 10.
The prior art circuit has at least two disadvantages. First, the use of discrete components 20 and 28 increases the manufacturing cost and space requirements for the protection circuit. Second, the discrete components exhibit a large ON resistance that represents a significant power loss for the circuit.
Several prior art inventions describe battery protection circuits. U.S. Pat. No. 6,037,750 to Von Novak describes a battery pack controller. Von Novak places the battery charging circuits (elements 206 and 208 of FIG. 2) inside the battery pack. Such an approach creates at least two significant problems. First, power consumption and heat build up from the charger may cause the connectivity to the external FET devices (202 and 204) to fail. Second, if the ground connection to the battery pack is lost, then control of the battery is lost. Battery charging may continue without control and may then result in a battery explosion. U.S. Pat. No. 5,781,390 to Notaro et al teaches an automotive battery protection circuit for protecting against reverse battery and over voltage. U.S. Pat. No. 5,896,025 to Yamaguchi et al describes a battery protection circuit. U.S. Pat. No. 5,789,900 to Hasegawa et al discloses a battery protection circuit showing two MOSFETs back-to-back. U.S. Pat. No. 6,160,381 to Peterzell discloses a battery pack protection circuit. Two MOSFET switches are used.
A principal object of the present invention is to provide an effective and very manufacturable circuit for protecting a battery.
A further object of the present invention is to provide a battery protection circuit with reduced manufacturing cost.
Another further object of the present invention is to provide a battery protection circuit with reduced space requirements.
Another further object of the present invention is to provide a battery protection circuit with reduced ON resistance in the safety circuit.
A still further object of the present invention is to provide a circuit that protects the battery even if the ground connection to the battery pack is lost.
In accordance with the objects of this invention, a battery protection circuit is achieved. The circuit comprises, first, a FET switch. Last, a control circuit determines the ON/OFF state of the FET switch. The FET switch and the control circuit comprise a single integrated circuit device. The control circuit may comprise over charge and over discharge detectors, a voltage reference, and a level shifter.